This is the blend (gradient) tool, The ability to paint a gradient directly onto the canvas is not new, but I do not know how long editing a gradient in place has been there. Adjusting the colour stops, now shows up straight away in the tool.
The gradient file - something.ggr does of course, have to be in your Gimp profile gradients folder, to be editable.

Developers are threatening to include this in an updated Gimp 2.8.(24?) sometime.